Influences of air curtain device on the droplet deposition characteristics of boom sprayers

Boom sprayers can deliver some droplets into the middle and lower strata of dense crop canopies, resulting in the uneven deposition and low pest control efficacy. Dense crops, such as the soybeans, form closed canopies in the middle and late growth stages, with the overlapping leaves blocking droplet penetration in field pest prevention. In this study, an air curtain device was developed to enhance the droplet penetration and uniform deposition in the dense soybean canopies. An air-curtain with boom sprayers was developed using Computational Fluid Dynamics (CFD) simulation. The device was composed of the conical duct structure with a total length of 27 m. 674 circular outlets (30 mm diameter, and 40 mm spacing), and a fillet at the fan-duct transition for the uniform distribution of static pressure. Grid division was performed on the 3D model, where the outlet area grid was refined to improve the simulation accuracy. The RNG k-ε turbulence model was selected for flow field analysis. Performance validation was conducted using a handheld anemometer (DLY-1603A) under different fan speeds. Field experiments were carried out in soybean fields, particularly with a leaf area index of 6.40 and an average plant height of 107 cm. The optimal air-curtain boom (spray pressure 1 062 kPa, and air volume 15 m³/s) was compared with a conventional boom equipped with XR8002, XR8004, XR8005 flat-fan nozzles and a JA3 hollow-cone nozzle. Water-sensitive and metal collectors were placed on the adaxial and abaxial leaf surfaces at 66 cm (mid-canopy) and 33 cm (lower canopy) to quantify the droplet density, coverage, and deposition. The data was also analyzed via threshold segmentation, weighing, and statistical tests. The transverse wind speed coefficient of variation (CV) of the air curtain outlet was below 3.35% under fan speeds of 1500 and 2000 r/min, indicating the high airflow uniformity. Compared with the conventional boom, the air-assisted system was achieved in the remarkable improvements: droplet density of 51 droplets/cm² (765% increase), surface coverage of 3.05% (293% increase), and liquid deposition of 0.24 μL/cm² (150% increase). Notably, the air-curtain sprayer also delivered the measurable abaxial coverage (0.66% in mid-canopy, and 0.32% in lower canopy) and droplet density (22 droplets/cm² in mid-canopy, and 37 droplets/cm² in lower canopy). While the conventional boom showed the nearly zero abaxial deposition—attributed to the airflow-induced leaf flipping for the high droplet accessibility to undersides. Spatial uniformity was significantly enhanced: The CV for droplet density was 54% (vs. 131%-141% for conventional), coverage CV was 107% (vs. 138%-156% for conventional), though deposition amount CV (135%) was slightly higher than the conventional sprayer (102%-109%). Nozzle type shared no significant effect on the droplet density or deposition in the influencing coverage; The JA3 nozzle was produced the finer droplets (118 μm) than the XR series (180-384 μm), thus aligning with the coverage variations. Pearson correlation analysis confirmed that there were significant positive correlations between coverage, density, and deposition (r=0.974-0.998, P<0.05). A transverse outlet-velocity CV at or below 3.50% was realized for the effective penetration and abaxial deposition in the dense soybean canopies. The air-curtain system was improved the droplet deposition, spatial uniformity, and leaf back accessibility, providing a practical solution to enhance pest control efficacy. The better deposition was attributed to the airflow-induced leaf flipping and uniform wind field distribution. Future research should explore the optimal coupling of fan speed, droplet size, and travel speed across different crops and growth stages. The findings can provide a strong reference to optimize the more complex canopy structures. Technical support can also improve the pesticide utilization and control in sustainable agriculture.